1. Field of the Invention
The present invention relates to a dose monitoring method for monitoring dose in a lithography process with improved accuracy, and a manufacturing method of a semiconductor device.
2. Description of the Related Art
Due to the demands for miniaturization of a device pattern of a semiconductor device, an exposure wavelength has been reduced, the NA of a projection lens has been raised, and processing techniques have improved. However, in recent years, demands for miniaturization of the device pattern have become stricter. As a result, it is difficult to obtain a sufficient dose tolerance and an exposure margin of a focal depth, and this has caused a drop in yield. Therefore, higher accuracy dose and focus control are needed in order to deal with the limited dose tolerance, to prevent a drop in yield.
The dose was previously judged by measurement of a line width of the pattern. However, the line width of the pattern changes not only with the dose but also with focus. If the pattern is made finer, the influence of a focus error on the line width of the pattern cannot be ignored. Therefore, it is difficult to judge whether the influence is caused by fluctuation of an adequate dose or by fluctuation of a focus position. Therefore, to realize exposure of the pattern of a minimum design rule with high accuracy, both the dose and the focus have been requested to be separately monitored with high accuracy, and to be individually managed.
As a method of measuring effective dose which is not influenced due to focus fluctuation, in SPIE Vol. 1261 Integrated Circuit Metrology, Inspection, and Process Control IV (1990) p. 315, a method has been proposed with respect to a dose monitor pattern which is not influenced by the line width by focus error. This dose monitor pattern is characterized in that a transmission portion and shield portion are arranged in one direction in a block having a width not resolved in a projection exposure apparatus for use. A plurality of blocks different in dimension ratio of the transmission portion to the shield portion are arranged in the direction while the dimension ratio is monotonously changed.
Upon illumination of the dose monitor pattern with illumination light, a latent image having an inclination distribution of an irradiation amount which does not depend on a state of focus is formed in a resist film on a substrate. When the length of one direction of the latent image formed in the resist film or the pattern obtained by developing the resist film is measured, the dose is measured.
Sensitivity of the dose monitor pattern is determined of fineness of setting of a duty ratio. When a width p of a block is large, and the duty ratio is fine, the dose monitor pattern can be formed with a higher sensitivity.
With refinement of the design rule, the exposure apparatus tends to be high in NA, short in wavelength, and high in σ. Therefore, the required width p of the block in the dose monitor pattern has tended to be finer. Furthermore, also for the duty ratio, in manufacturing a mask, it is difficult to set a pattern increment in design to be extremely fine. Due to the above-described reason, it has been difficult to obtain the sensitivity of the effective dose, required for the dose monitor pattern.
For example, it is assumed that a mask to be monitored in the effective dose is set in the exposure apparatus having exposure conditions including a numerical aperture NA of 0.68, a coherent factor σ of 0.85, and a wavelength λ of 0.193 μm. The condition of the width p (dimension on a wafer) of the block which is not resolved in this apparatus is as follows according to diffraction theory:                                                                         1                P                            ≥                            ⁢                                                (                                      1                    +                    σ                                    )                                ⁢                                  NA                  λ                                                                                                        P              ≤                            ⁢                              λ                                  NA                  ⁡                                      (                                          1                      +                      σ                                        )                                                                                                          (        1        )            From the equation (1), it is necessary to set the width p to 0.15 μm or less. It is a difficult situation to prepare a block having a width of 0.15 μm or less with high accuracy. Moreover, to raise detection sensitivity of the dose monitor pattern, the duty ratio has been requested to be set to be as fine as possible, and this is further difficult in manufacturing the pattern.
As described above, it is difficult to manufacture the dose monitor pattern with high a accuracy. As a result, there is a problem that it is difficult to obtain the sensitivity of the effective dose which has been required for the dose monitor pattern.